13C NMR chemical shift trends and conformational changes in alkali metal cation complexes of a series of naphthalene crown ethers

13C NMR studies of alkali metal cation complexation by six related crown ethers are described. The crown ethers (and ions studied) were 2,3‐naphtho‐20‐crown‐6 (Na+, K+, Rb+, Cs+), 2,3‐naphtho‐17‐crown‐5 (Na+, K+, Rb+, Cs+), 2,3‐naphtho‐14‐crown‐4 (Na+, K+, Rb+, Cs+), 1,8‐naphtho‐21‐crown‐6 (Na+, K+, Rb+, Cs+), 1,8‐naphtho‐18‐crown‐5 (Na+, K+, Rb+, Cs+) and 1,5‐naphtho‐22‐crown‐6 (K+, Rb+, Cs+). For each crown ethercation combination, 13C NMR spectra were obtained for a number of different ion concentrations in CD3OD. Data were analysed by a non‐linear least‐squares treatment to give complexation constants and chemical shifts that would result at the limit of complete 1:1 complexation. Analyses of the limiting chemical shifts for a number of carbons indicate regular, cation‐dependent trends (for Cs+ through Na+). Deviations from these trends are interpreted as reflecting differences in conformations of crown ether–cation complexes. Conformational differences between cation complexes that occur near the point of naphthalene attachment cause deviations from the trend in the aromatic carbon chemical shifts, while conformational differences further from the naphthalene cause trend deviations only in the crown ether carbon chemical shifts.